Method of removing silicon nitride film

ABSTRACT

For selectively removing a silicon nitride film formed on a bottom of a contact hole or the like in a semiconductor device, plasma etching is performed using a process gas supplied therefor which is comprised of a first fluorine compound including a carbon atom-carbon atom bond [for example, octafluorocyclobutane (C 4 F 8 ), hexafluorobutadiene (C 4 F 6 ), octafluorocyclopentene (C 5 F 8 )], and a second fluorine compound including at least one hydrogen atom and a single carbon atom in one molecule (for example, fluoromethane (CH 3 F) , difluoromethane (CH 2 F 2 ), trifluoromethane (CHF 3 )]. According to this method, the silicon nitride film on the bottom can be selectively removed without removing a silicon nitride film formed on a side wall of the contact hole and the like.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of fabricating asemiconductor device, and more particularly, to a method of removing asilicon nitride film formed on the bottom of a contact hole and the likein fabrication of a semiconductor device.

[0003] 2. Description of the Related Art

[0004] With the miniaturization in fabrication processes ofsemiconductor devices, the miniaturization is also under way in contactholes, via holes and the like formed through interlayer insulating filmsin semiconductor devices. Since interlayer insulating films cannot bereduced in thickness in concert with the progress of miniaturization indesign rules, the aspect ratios of holes such as contact holesnecessarily become larger. In addition, due to a requirement forreducing variations in alignment of contact holes with underlying wiresand the like, associated with the miniaturization in the fabricatingprocess, a self-aligned contact (SAC) process has drawn more and moreattention because this process can eliminate a design margin for thealignment on a photomask.

[0005] While there are several SAC processes, a typical one involvesforming a gate electrode and a gate wire, an offset oxide film disposedon the top faces of them, and side walls (oxide films) disposed on sidefaces of the gate electrode and gate wire, conformally forming a thinSiN (silicon nitride) film over the entire surface as an etching stopperfilm, subsequently forming an interlayer insulating film (oxide film),and selectively removing the interlayer insulating film at and nearpositions at which contact holes are to be formed through aphotolithographic step. In this event, since the final positions of thecontact holes are determined by the side walls and offset oxide film,this can be said a sort of self-aligned process. Finally, the SiN filmis removed from the bottoms of the contact holes which are then filledwith contact plugs.

[0006] Further, in the SAC process, an attempt has been made to use anSiN film instead of the offset oxide film formed on the gate electrodeand gate wire, and to use an SiN film as well for the side walls.

[0007] Depending on a process which follows the formation of contactholes, the interlayer insulating film, which is an oxide film, can bedamaged. For protecting the interlayer insulating film from such adamage, a thin SiN film may be formed on a side wall (inner wall) of aformed contact hole, for example, in thickness of 10 to 20 nm. In thiscase, the SiN film on the bottom of the contact hole must be removedafter the SiN film is formed on the side wall of the contact hole.Particularly, the formation of such SiN film is deemed as essential whena relatively “soft” oxide such as BPSG (borophosphosilicate glass) isused as the interlayer insulating film.

[0008]FIG. 1 is a schematic cross-sectional view illustrating a contacthole before an SiN film is removed after the contact hole was formed. Apair of wiring patterns 12 or electrode patterns made of WSi (tungstensilicide) are formed on substrate 11 made of silicon or the like, andsimilarly patterned offset SiN films 13 are formed on wiring patterns12. Further, side walls 14 similarly made of SiN are provided on sidefaces of wiring patterns 12 and offset SiN films 13. Then, interlayerinsulating film 15 made of silicon oxide is formed over the entiresurface of substrate 11 including wiring patterns 12, offset SiN films13 and side walls 14. Interlayer insulating film 15 is formed with acontact hole 16 by an SAC process. Contact hole 16 extends throughinterlayer insulating film 15 to the surface of substrate 11 in a regionsandwiched by the pair of wiring patterns 12.

[0009] Thin SiN film 17 is formed on the bottom and inner side face(side wall) of contact hole 16. Here, the bottom of contact hole 16refers to a portion of the contact hole which is in contact with thesurface of substrate 11. This SiN film 17 is provided as a film forprotecting interlayer insulating film 15 from a wet etching and the likein subsequent processes. While SiN film 17 is also formed on the topface of interlayer insulating film 15 depending on its depositionprocess, the SiN film overlying interlayer insulating film 15 may beremoved as required by a subsequent CMP (chemical mechanical polishing)process or the like. It should be noted that contact hole 16 formed bythe SAC process is generally set such that its diameter on the top faceof interlayer insulating film 15 is larger than that on the bottom ofcontact hole 16. Therefore, the diameter of contact hole 16 on thebottom thereof is determined by side walls 14 in a self-aligned manner,and shoulder 18 is formed within contact hole 16.

[0010] When contact hole 16 is used for interlayer connection, SiN film17 must be removed from the bottom of contact hole 16, as describedabove, before contact hole 16 is filled with a wiring material or awiring plug. In this event, since the SiN film on the side wall ofcontact hole 16 must be left, anisotropic etching is used. Suchanisotropic etching may be dry etching such as plasma etching.

[0011] When the SiN film on the bottom of contact hole 16 is removed byplasma etching, a gas system conventionally used for this purpose is agas system of CHF₃/Ar/O₂, a gas system of CH₂F₂/Ar/O₂, and the like.When the former gas system is used, an etching reaction is expressed by:

Si₃N₄+4CHF₃→3SiF₄↑+4HCN ↑

[0012] In this example, a product having a relatively high vaporpressure, such as SiF₄, HCN or the like is formed to promote the etchingreaction. Likewise, with the latter gas system a product having arelatively high vapor pressure is produced such as SiF₄, HCN or thelike.

[0013] However, the dry etching which uses the aforementionedconventional gas system has a problem in that the SiN film on the sideface of the contact hole, as well as the SiN film on the bottom of thecontact hole is inevitably etched to some degree. Since etching is lessadvanced as the aspect ratio is larger, in other words, since an upperportion of a contact hole, i.e., a region near the entrance of thecontact hole is etched in advance, the etching rate at the shoulderwithin the contact hole is higher than the etching rate on the bottom ofthe contact hole. In some cases, as illustrated in FIG. 2, an SiN filmon side wall 14 has been etched away before the SiN film on the bottomof contact hole 16 is completely removed, causing wiring patterns 13made of WSi to expose to contact hole 16. If contact hole 16 is embeddedwith a contact plug metal with exposed wiring patterns 13, the contactplug will be short-circuited with wiring patterns 13.

SUMMARY OF THE INVENTION

[0014] It is therefore an object of the present invention to provide amethod of removing a silicon nitride (SiN) film which is capable ofreliably removing an SiN film on the bottom of a contact hole withoutremoving an SiN film formed on a side face of the contact hole, even ifthe contact hole has a large aspect ratio.

[0015] The inventor diligently repeated investigations for achieving theabove object, and as a result found that a silicon nitride film on thebottom of a hole such as a contact hole alone can be selectively removedby using a process gas which comprises a first fluorine compoundincluding a carbon atom-carbon atom bond, and a second fluorine compoundincluding at least one hydrogen atom and a single carbon atom in onemolecule, thereby completing the present invention. Assume in thepresent invention that a double bond C═C and a triple bond C≡C also fallunder the carbon atom-carbon atom bond, in addition to the single bondC—C. In the present invention, preferably used as the first fluorinecompound may be, by way of example, octafluorocyclobutane (C₄F₈),hexafluorobutadiene (C₄F₆), octafluorocyclopentene (C₅F₈) and the like.On the other hand, preferably used as the second fluorine compound maybe, by way of example, monofluoromethane (CH₃F), difluoromethane(CH₂F₂), trifluoromethane (CHF₃) and the like.

[0016] Specifically, a method of removing a silicon nitride filmaccording to the present invention has an application in removal of asilicon nitride film formed on a surface of a material. The methodincludes the steps of supplying a process gas which comprises a firstfluorine compound having fluorine atoms and at least two carbon atoms inone molecule, and a second fluorine compound having at least onefluorine atom, at least one hydrogen atom and a single carbon atom inone molecule, and performing dry etching using the process gas to removethe silicon nitride film.

[0017] In the following description, for facilitating the understandingthe invention by comparison in terms of chemical structures, the firstfluorine compound, i.e., a fluorine compound including a carbonatom-carbon atom bond may be called the “higher-order fluorocarbon” andthe second fluorine compound, i.e., a fluorine compound including atleast one hydrogen atom and a single carbon atom in one molecule, the“lower-order fluorocarbon.”

[0018] Conventionally, higher-order fluorocarbons such asoctafluorocyclobutane (C₄F₈), hexafluorobutadiene (C₄F₆),octafluorocyclopentene (C₅F₈) and the like have been used for etchingsilicon oxide films, particularly for removing a silicon oxide filmformed on the bottom of a hole having a large aspect ratio, such as acontact hole. However, due to a low vapor pressure of a product obtainedby a reaction with silicon nitride, and high probability of producingdeposition, the higher-order fluorocarbons are deemed as substantiallyfailing to provide etching performance for a silicon nitride film, sothat the higher-order fluorocarbons have not been used for etching asilicon nitride film in any examples.

[0019] However, the inventor has found that a silicon nitride film alonecan be selectively removed on the bottom of a hole such as a contacthole by using such a higher-order fluorocarbon in combination with alower-order fluorocarbon such as fluoromethane (CH₃F), difluoromethane(CH₂F₂), trifluoromethane (CHF₃) or the like. The lower-orderfluorocarbon is conventionally used for etching silicon nitride films.Specifically, the inventor has found that a silicon nitride film formedon the bottom of a contact hole or the like can be completely removedwithout etching a silicon nitride film formed on a side wall of thecontact hole or the like by supplying a process gas including ahigher-order fluorocarbon and a lower-order fluorocarbon, and performingdry etching using this process gas.

[0020] As a mechanism of the foregoing phenomenon, the inventor assumesa process as follows: As a higher-order fluorocarbon is decomposedwithin a plasma, C_(x)F_(y) radicals, derived from the higher-orderfluorocarbon, increase in the vapor phase. The C_(x)F_(y) radicals,however, can introduce shallow into a hole, but not deep into the holedue to the adsorption characteristic thereof. On the other hand,radicals derived from a lower-order fluorocarbon introduce deep into thehole to etch an SiN film. Thus, the bottom of the contact hole is etchedby the radicals derived from the lower-order fluorocarbon, while a sidewall and a shoulder of the contact hole are protected by an inversemicro-loading effect of the C_(x)F_(y) radicals derived from thehigher-order fluorocarbon. In this manner, the SiN film alone isselectively etched on the bottom of the hole. The inverse micro-loadingeffect, herein referred to, means that a location shallow in a hole isnot etched but a location deep in the hole is etched.

[0021] The inventor further investigated a preferred ratio of thehigher-order fluorocarbons (first fluorine compounds) to the lower-orderfluorocarbons (second fluorine compounds) in a supplied process gas, andeventually found that the following relationship is preferablysatisfied:

1<R₁/R₂<4

[0022] where R₁ is the sum of m₁×n_(C−C) calculated for the respectivefirst fluorine compounds; R₂ is the sum of m₂×n_(C−H) calculated for therespective second fluorine compounds; n_(C−C) is the number of carbonatom-carbon atom bonds included in one molecule of each first fluorinecompound; m₁ is a mole fraction of each first fluorine compound in thesupplied process gas; n_(C−H) is the number of carbon atom-hydrogen atombonds included in one molecule of each second fluorine compound; and m₂is a mole fraction of each second fluorine compound in the suppliedprocess gas.

[0023] The method of removing a silicon nitride film according to thepresent invention is particularly suitable for removing a siliconnitride film formed on the bottom of a hole such as a contact holeformed in fabrication of a semiconductor device. Such a hole may be ahole, for example, having an aspect ratio in a range of three to ten.

[0024] The above and other objects, features, and advantages of thepresent invention will become apparent from the following descriptionwith reference to the accompanying drawings which illustrate examples ofthe present invention.

BRIEF DESCRIPTION OF DRAWINGS

[0025]FIG. 1 is a schematic cross-sectional view illustrating a contacthole formed by an SAC (self-aligned contact) process;

[0026]FIG. 2 is a schematic cross-sectional view illustrating a contacthole in which a silicon nitride film on a side wall is etched;

[0027]FIG. 3 is a schematic cross-sectional view illustrating a contacthole in which a silicon nitride film is etched in accordance with amethod of the present invention;

[0028]FIG. 4 is a schematic cross-sectional view illustrating thestructure of a sample for evaluation in Example 1;

[0029]FIGS. 5A to 5D are graphs for explaining the relationship betweenan etching rate on the bottom and an etching rate at a shoulder whenCHF₃/Ar/O₂/C₄F₈ is used as a gas system;

[0030]FIGS. 6A to 6D are graphs for explaining the relationship betweenan etching rate on the bottom and an etching rate at a shoulder whenCH₂F₂/Ar/O₂/C₄F₈ is used as a gas system;

[0031]FIGS. 7A to 7D are graphs for explaining the relationship betweenan etching rate on the bottom and an etching rate at a shoulder whenCHF₃/Ar/O₂/C₅F₈ is used as a gas system;

[0032]FIG. 8 is a schematic cross-sectional view illustrating thestructure of a device in Example 2; and

[0033]FIG. 9 is a schematic cross-sectional view illustrating thestructure of a device in Example 3.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Next, the present invention will be described in detail bydiscussing the removal of a silicon nitride film formed on the bottom ofa hole such as a contact hole formed in fabrication of a semiconductordevice.

[0035] First, an interlayer insulating film is formed on a semiconductorsubstrate by a known semiconductor fabricating process, and a contacthole is formed by an SAC process. In this event, an SiN (siliconnitride) film may be previously formed at a position, at which thebottom of the contact hole is defined, for use as an etching stopperfilm when the contact hole is formed. Next, a thin SiN film is formed onan inner wall and bottom of the contact hole. The cross-sectional shapeat this time is illustrated in the aforementioned FIG. 1.

[0036] Next, the semiconductor substrate formed with the contact hole inthe foregoing manner is placed in a reaction chamber for performingplasma etching. A process gas is supplied into the reaction chamberafter the reaction chamber is decompressed to a predetermined pressure.An RF (radio frequency) discharge is generated in the reaction chamberwhile adjusting the amount of the process gas supplied to the reactionchamber and the amount of gas exhausted from the reaction chamber tomaintain a constant pressure within the reaction chamber, and the plasmaetching is performed. A process gas used in this event is a mixture of afirst fluorine compound having a carbon atom-carbon atom bond, forexample, octafluorocyclobutane (C₄F₈), hexafluorobutadiene (C₄F₆),octafluorocyclopentene (C₅F₈) or the like, and a second fluorinecompound including at least one hydrogen atom and a single carbon atomin one molecule, for example, monofluoromethane (CH₃F), difluoromethane(CH₂F₂), trifluoromethane (CHF₃) or the like. The process gas mayadditionally include gases such as argon (Ar), oxygen (O₂) and the like.The process gas is composed such that the sums R₁, R₂ satisfy thefollowing relationship:

1≦R₁/R₂≦4  (1)

[0037] where R₁ is the sum of m₁×n_(C−C) calculated for the respectivefirst fluorine compounds; R₂ is the sum of m₂×n_(C−H) calculated for therespective second fluorine compounds; m₁ is a mole fraction of eachfirst fluorine compound in the supplied process gas; m₂ is a molefraction of each second fluorine compound in the supplied process gas;n_(C—C) is the number of carbon atom-carbon atom bonds (i.e., C—C, C═Cand C≡C bonds) included in one molecule of each first fluorine compound;and n_(C—H) is the number of carbon atom-hydrogen atom bonds (i.e., C—Hbond) included in one molecule of each second fluorine compound.

[0038] Here, when the process gas is composed of one of the firstfluorine compounds and one of the second fluorine compounds, thefollowing relationship may be satisfied:

1≦m₁×n_(C—C)/(m₂×n_(C—H))≦4

[0039] Of course, a plurality of the first fluorine compounds may beused in combination. Similarly, a plurality of the second fluorinecompounds may be used in combination.

[0040] When the plasma etching is performed under the foregoingcondition, the silicon nitride film on the bottom of contact hole 16 iscompletely removed without etching the side wall of contact hole 16, ascan be seen in the cross-sectional shape illustrated in FIG. 3. In thisevent, since shoulder 18 within contact hole 16 is positioned above thebottom of the hole, higher-order fluorocarbons adsorb on the surface ofshoulder 18 to limit the etching rate.

EXAMPLES

[0041] In the following, the present invention will be described ingreater detail in connection with examples.

EXAMPLE 1 Experiment For Confirming Basic Characteristics

[0042] Samples for evaluation were provided as illustrated in FIG. 4,and an SiN film of the evaluation sample was etched, while varying thecomposition and flow rate of a process gas as listed below, using acommercially available two-frequency parallel flat plate plasma etchingapparatus (frequencies: 60 MHz and 2 MHz) under conditions that thereaction chamber was maintained at a pressure of 4 Pa (30 mTorr), and RFpower of 1200 W/200 W was applied:

[0043] (Sample 1) Components: CHF₃/Ar/O₂/C₄F₈

[0044] Flow Rate: X/600/15/Y (unit: sccm)

[0045] (Sample 2) Components: CH₂F₂/Ar/O₂/C₄F₈

[0046] Flow Rate: X/600/15/Y (unit: sccm)

[0047] (Sample 3) Components: CHF₃/Ar/O₂/C₅F₈

[0048] Flow Rate: X/600/15/Y (unit: sccm)

[0049] Then, the etching rate on bottom 21 of hole 20 and the etchingrate at shoulder 22 were measured under the respective etchingconditions to calculate the ratio of the rate on the bottom (bottomrate) to the rate at the shoulder (shoulder rate) for each sample.Further, dependency of the bottom rate/shoulder rate on the X/Y ratiowas examined under each etching condition to find an optimal X/Y ratio.

[0050]FIGS. 5A to 5D show the results on Sample 1; FIGS. 6A to 6D showthe results on Sample 2; and FIGS. 7A to 7D show the results on Sample3. Here, X corresponds to a mole flow rate of a lower-orderfluorocarbon, that is, second fluorine compound, while Y corresponds toa mole flow rate of a higher-order fluorocarbon, that is first fluorinecompound. In these graphs, FIGS. 5A, 6A and 7A show the etching rates onthe bottom and at the shoulder; FIGS. 5B, 6B and 7B show the etchingrates at the bottom and at the shoulder in a region where the proportionof the higher order fluorocarbons is small; FIGS. 5C, 6C and 7C show thebottom rate/shoulder rate; and FIGS. 5D, 6D and 7D show the bottomrate/shoulder rate in a region where the proportion of the higher orderfluorocarbons is small. In either of the graphs, the horizontal axisrepresents Y/X.

[0051] As can be seen from FIGS. 5A to 5D, with the process gascomprised of CHF₃/Ar/O₂/C₄F₈, the etching rate at the shoulder isreduced as a C₄F₈/CHF₃ flow ratio is higher. It seems that this isbecause deposition produced from C₄F₈ prevents the silicon nitride filmfrom the etching. When a small amount of C₄F₈ was added, the etchingrate on the bottom slightly increases. It seems that this is becauselower-order fluorocarbons produced from C₄F₈, on the contrary, promotethe etching of the silicon nitride film on the bottom of the hole towhich the deposition is difficult to access. The etching rate on thebottom exhibits the highest value when the C₄F₈/CHF₃ flow ratio (moleratio) is 0.25, and subsequently becomes lower together with the etchingrate at the shoulder. Further, it was found through an observation thatthe etching was not advanced partially on the bottom of the hole in aregion of C₄F₈/CHF₃>1 (etching stop state). In FIGS. 5A to 5D, a pointon the horizontal axis indicating the C₄ F₈/CHF₃ flow ratio equal to0.25 corresponds to R₁/R₂ ₌1 in Equation (1), and a point indicating theflow ratio equal to 1.0 corresponds to R₁/R₂ ₌4.

[0052] A similar tendency was found also in the process gas comprised ofCH₂F₂/Ar/O₂/C₄F₈ shown in FIGS. 6A to 6D, and the process gas comprisedof CHF₃/Ar/O₂/C₅F₈ shown in FIGS. 7A to 7D. In FIGS. 6A to 6D, a pointon the horizontal axis indicating the C₄F₈/CH₂F₂ flow ratio equal to 0.5corresponds to R₁/R₂=1 in Equation (1), and a point indicating the flowratio equal to 2.0 corresponds to R₁/R₂=4. In FIGS. 7A to 7D, a point onthe horizontal axis indicating the C₅F₈/CHF₃ flow ratio equal to 0.2corresponds to R₁/R₂ ₌1 in Equation (1), and a point indicating the flowratio equal to 0.8 corresponds to R₁/R₂ ₌4.

[0053] In conclusion, irrespective of the type of lower-orderfluorocarbons or higher-order fluorocarbons, when higher-orderfluorocarbons are not at all included, i.e., when only lower-orderfluorocarbons are included as a fluorine compound, the etching rate atthe shoulder is significantly higher than the etching rate at thebottom, causing prominent side etching as described in the backgroundart. As more higher-order fluorocarbons are added, the etching rate atthe shoulder consistently decreases. On the other hand, the etching rateon the bottom once increases to substantially the same as the etchingrate at the shoulder, and subsequently decreases in a manner similar tothe etching rate at the shoulder.

[0054] As a result, it was found that conditions that the etching rateat the shoulder is substantially equal to the etching rate at thebottom, and the etching stop state is avoided, are expressed by:

1≦R₁/R₂≦4

[0055] where R₁ is the sum of m₁×n_(C—C) calculated for the respectivechemical species included in the higher-order fluorocarbons; R₂ is thesum of m₂×n_(C—H) calculated for the respective chemical speciesincluded in the lower-order fluorocarbons; n_(C—C) is the number ofcarbon atom-carbon atom bonds included in one molecule of each chemicalspecies of the higher-order fluorocarbons; m₁ is a mole fraction of eachchemical species of the higher-order fluorocarbons in the suppliedprocess gas; n_(C—H) is the number of carbon atom-hydrogen atom bondsincluded in one molecule of each chemical species of the lower-orderfluorocarbons; and m₂ is a mole fraction of each chemical species of thelower-order fluorocarbons.

EXAMPLE 2 Exemplary Application To Real Device

[0056] The method of the present invention was applied to a real devicemanufactured by an SAC process. Here, a real device was fabricated indimensions shown in FIG. 8, and a silicon nitride film was removed fromthe bottom of a contact hole (the aspect ratio of which is 4.7) in thisreal device. Using a commercially available two-frequency parallel flatplate plasma etching apparatus (frequencies: 60 MHz and 2 MHz) underconditions that the reaction chamber was maintained at a pressure of 4Pa (30 mTorr), and RF power of 1200 W/200 W was applied, CHF₃/Ar/O₂/C₄F₈was used as a process gas, and their flow rates were chosen to be 15sccm/600 sccm/15 sccm/15 sccm. Then, the amount of etched siliconnitride at a shoulder was measured in the event of 30% overetching. As aresult, the amount of etched shoulder was 23 nm. Assuming that theetching rate at the shoulder was equal to the etching rate at thebottom, the amount of etched silicon nitride film at the shoulder shouldbe 26 nm, so that it was found from the etching rate measured in thisexample that the etching rate was higher on the bottom than at theshoulder.

EXAMPLE 3 Exemplary Application To Real Device

[0057] The method of the present invention was applied to a real devicehaving a miniature contact hole. Here, a real device was fabricated indimensions shown in FIG. 9, and a silicon nitride film was removed fromthe bottom of a contact hole (the aspect ratio of which is 8.7) in thereal device. Using a commercially available two-frequency parallel flatplate plasma etching apparatus (frequencies: 60 MHz and 2 MHz) underconditions that the reaction chamber was maintained at a pressure of 4Pa (30 mTorr), and RF power of 1200 W/200 W was applied, CHF₃/Ar/O₂/C₄F₈was used as a process gas, and their flow rates were chosen to be 15sccm/600 sccm/15 sccm/15 sccm. Then, the amount of etched siliconnitride on a side wall of the contact hole was measured in the event of30% overetching. As a result, the silicon nitride film on the bottom ofthe hole was completely etched away in the 30% overetching, and theamount of etched side wall was 0 nm in that event, thus eliminating sideetching. Actually, an emission spectrometer was used to monitor light atwavelength of 388 nm, and a just etching time was defined at a point atwhich the waveform was completely stabilized. Even at this just etchingtime, the silicon nitride film was completely etched on the bottom ofthe hole.

[0058] While preferred embodiments of the present invention have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the appendedclaims.

What is claimed is:
 1. A method of removing a silicon nitride filmformed on a surface of a material, comprising the steps of: supplying aprocess gas which comprises a first fluorine compound including a carbonatom-carbon atom bond, and a second fluorine compound including at leastone hydrogen atom and a single carbon atom in one molecule; andperforming dry etching using said process gas to remove said siliconnitride film.
 2. A method according to claim 1, wherein said firstfluorine compound is at least one of octafluorocyclobutane (C₄F₈),hexafluorobutadiene (C₄F₆), octafluorocyclopentene (C₅F₈).
 3. A methodaccording to claim 1, wherein said dry etching is plasma etching.
 4. Amethod according to claim 1, wherein said second fluorine compound is atleast one of monofluoromethane (CH₃F), dif luoromethane (CH₂F₂), andtrifluoromethane (CHF₃).
 5. A method according to claim 2, wherein saidsecond fluorine compound is at least one of monofluoromethane (CH₃F),difluoromethane (CH₂F₂), and trifluoromethane (CHF₃).
 6. A methodaccording to claim 5, wherein said dry etching is plasma etching.
 7. Amethod according to claim 1, wherein: following relationship issatisfied: 1≦R₁/R₂≦4 where R₁ is a sum of m₁×n_(C—C) calculated for saidrespective first fluorine compounds; R₂ is a sum of m₂×n_(C—H)calculated for said respective second fluorine compounds; n_(C—C) isnumber of carbon atom-carbon atom bonds included in one molecule of eachsaid first fluorine compound; m₁ is a mole fraction of each said firstfluorine compound in said supplied process gas; n_(C—H) is number ofcarbon atom-hydrogen atom bonds included in one molecule of each saidsecond fluorine compound; and m₂ is a mole fraction of each said secondfluorine compound in said supplied process gas.
 8. A method according toclaim 5, wherein: the following relationship is satisfied: 1<R₁/R₂<4where R₁ is a sum of m₁×n_(C—C) calculated for said respective firstfluorine compounds; R₂ is a sum of m₂×n_(C—H) calculated for saidrespective second fluorine compounds; n_(C—C) is number of carbonatom-carbon atom bonds included in one molecule of each said firstfluorine compound; m₁ is a mole fraction of each said first fluorinecompound in said supplied process gas; n_(C—H) is number of carbonatom-hydrogen atom bonds included in one molecule of each said secondfluorine compound; and m₂ is a mole fraction of each said secondfluorine compound in said supplied process gas.
 9. A method according toclaim 1, wherein said surface of the material is a bottom of a contacthole formed in a semiconductor device.
 10. A method according to claim6, wherein said surface of the material is a bottom of a contact holeformed in a semiconductor device.
 11. A method according to claim 1,wherein said surface of the material is a bottom of a hole having anaspect ratio in a range of three to ten.
 12. A method according to claim6, wherein said surface of the material is a bottom of a hole having anaspect ratio in a range of three to ten.